1. Field of the invention:
The present invention relates to an equal waveguide used in an image sensor of a facsimile and an image sensor using the same.
2. Description of the prior art:
Conventional optical systems used for an image input process in a facsimile employ a distributed index lens array (e.g., SELFOC LENS ARRAY (SLA); trade name), an optical fiber array, or the like. The optical system employing the distributed index lens array is expensive because of its complicated production process, and that employing the optical fiber array has problems in its illumination process or the like.
Now, referring to FIGS. 5, 6, and 7, description will be given for the conventional optical system employing an optical fiber array used for an image input process. FIG. 5 shows a conventional fiber sheet used as the optical system which comprises cores 1, claddings 2, and absorbing layers 3. In this fiber sheet, although the absorbing layers 3 prevent interfering light or leakage light from deteriorating resolution or contrast of an image of an original, complicated process for arranging fibers in a proper manner leads to an increase in cost.
FIG. 6 shows another conventional fiber sheet which has been proposed in order to decrease the cost of the conventional fiber sheet of FIG. 5 (see, Japanese Laid-open Patent Publication No. 54-88143). The fiber sheet of FIG. 6 comprises cores 4, claddings 5 and scattering elements 6. This fiber sheet is produced in such a manner that a photo mask is disposed on a macromolecular film sheet impregnated with polymeric monomers, which is exposed to light or irradiated by an electron beam, or scanned with light or an electron beam irradiated from an array of optical sources generating a point irradiation beam, so that monomers are selectively polymerized and then unreacted monomers are removed. In this process, a lot of scattering elements 6 are produced within the claddings 5 by control of the polymerization conditions, heat treatment or the like. Therefore, the fiber sheet of FIG. 6 can be produced at relatively low cost and deterioration in resolution or contrast of the image by the interfering light or the leakage light can be avoided.
However, in the fiber sheet having above-discussed construction of FIG. 6, incident light on a side A of each core 4 is directed to a side B while performing total reflection within the cores 4, without leaking out in a direction shown by an arrow X. However, since the light leaks out in a direction shown by an arrow Z, transmission efficiency may be reduced. In order to improve the transmission efficiency, cladding layers 5a and 5b are required to be formed on the upper and lower surfaces of the fiber sheet, respectively, as shown in FIG. 7. However, on the cladding layers 5a and 5b, light-shielding layers for shielding the interfering light (i.e., external light and light from a light source) must be disposed, which results in that the incident light can hardly reach the surface of an original.